U.S. patent number 8,444,942 [Application Number 13/362,025] was granted by the patent office on 2013-05-21 for process for removing contaminants from gas streams.
This patent grant is currently assigned to Linde Aktiengesellschaft. The grantee listed for this patent is Naresh Suchak. Invention is credited to Naresh Suchak.
United States Patent |
8,444,942 |
Suchak |
May 21, 2013 |
Process for removing contaminants from gas streams
Abstract
The present invention provides for process for inhibiting the
levels of nitrogen oxides in process gas streams from sulfuric acid
regeneration and sulfuric acid production plants. Partial oxidation
of the nitrogen oxides is achieved by feeding a sub stoichiometric
amount of ozone as to nitrogen oxides to the process gas
stream.
Inventors: |
Suchak; Naresh (Glen Rock,
NY) |
Applicant: |
Name |
City |
State |
Country |
Type |
Suchak; Naresh |
Glen Rock |
NY |
US |
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Assignee: |
Linde Aktiengesellschaft
(Munich, DE)
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Family
ID: |
46603049 |
Appl.
No.: |
13/362,025 |
Filed: |
January 31, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130028821 A1 |
Jan 31, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61438293 |
Feb 1, 2011 |
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Current U.S.
Class: |
423/210; 423/523;
423/522; 423/215.5; 423/235 |
Current CPC
Class: |
B01D
53/56 (20130101); B01D 53/60 (20130101); B01D
47/06 (20130101); C01B 17/765 (20130101); Y02A
50/20 (20180101); B01D 2251/104 (20130101) |
Current International
Class: |
B01D
53/14 (20060101); C01B 17/69 (20060101); B01D
53/75 (20060101); B01D 53/74 (20060101); B01D
53/56 (20060101); C01B 17/90 (20060101) |
Field of
Search: |
;423/210,215.5,235,522,523 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Vanoy; Timothy
Attorney, Agent or Firm: Von Neida; Philip H.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims priority from U.S. Provisional Patent
Application Ser. No. 61/438,293, filed Feb. 1, 2011.
Claims
Having thus described the invention, what I claim is:
1. A method for removing contaminants from a process gas stream in
a sulfuric acid production process comprising the steps: a)
directing the process gas stream to a particulate scrubber; b)
contacting the process gas stream with ozone at less than
stoichiometric amounts; c) scrubbing partially oxidized nitrogen
oxides in the particulate scrubber; d) establishing conditions
under which nitrous acid is formed and absorbed; e) feeding a
compound selected from the group consisting of urea, ammonia and a
compound that can provide an ammonical radical to the aqueous phase
of the particulate scrubber; e) directing the process gas stream to
a gas dryer; and f) recovering the process gas stream.
2. The method as claimed in claim 1 wherein said contaminants are
selected from the group consisting of sulfur oxides and nitrogen
oxides.
3. The method as claimed in claim 1 wherein said less than
stoichiometric amount is one-third the amount of ozone necessary to
oxidize nitrogen oxides.
4. The method as claimed in claim 1 wherein said conditions under
which nitrous acid is formed and absorbed are in the particulate
scrubber.
5. The method as claimed in claim 1 wherein said partial oxidation
is performed in a process gas stream selected from the group
consisting of upstream, within and downstream of a particulate
scrubber.
6. The method as claimed in claim 1 wherein said process gas stream
and ozone are mixed prior to the process gas stream entering the
particulate scrubber.
7. The method as claimed in claim 1 wherein said process gas stream
is quenched prior to mixing with ozone.
8. The method as claimed in claim 1 wherein air is injected into
said process gas stream prior to entering said gas dryer.
9. The method as claimed in claim 1 wherein said absorption of
nitrous acid is in a neutral or acidic medium.
10. The method as claimed in claim 1 further comprising a different
gas-liquid contacting device with aqueous scrubbing medium.
11. The method as claimed in claim 1 wherein said recovered process
gas stream is oxidized using catalytic converters and then fed to a
sulfuric acid absorption tower.
12. The method as claimed in claim 1 wherein said ozone is up to 10
weight percent ozone in oxygen.
13. A method for removing contaminants from a process gas stream
from a sulfuric acid production process comprising the steps: a)
feeding the process gas stream to a particulate scrubber; b)
introducing ozone into the process gas stream at less than
stoichiometric amounts with the oxides of nitrogen and scrubbing
the partially oxidized oxides of nitrogen in the scrubber; c)
establishing conditions in the scrubber to facilitate the formation
of nitrous acid; d) feeding a compound selected from the group
consisting of urea, ammonia and a compound that can provide an
ammonical radical to the aqueous phase of the particulate scrubber;
e) absorbing nitrous acid in a neutral or acidic medium; and f)
recovering the process gas stream.
14. The method as claimed in claim 13 wherein said contaminants are
selected from the group consisting of sulfur oxides and nitrogen
oxides.
15. The method as claimed in claim 13 wherein said less than
stoichiometric amount is one-third the amount of ozone necessary to
oxidize nitrogen oxides.
16. The method as claimed in claim 13 wherein said conditions under
which nitrous acid is formed and absorbed are in the particulate
scrubber.
17. The method as claimed in claim 13 wherein said partial
oxidation is performed in a process gas stream selected from the
group consisting of upstream, within and downstream of a
particulate scrubber.
18. The method as claimed in claim 13 wherein said process gas
stream and ozone are mixed prior to the process gas stream entering
the particulate scrubber.
19. The method as claimed in claim 13 wherein said process gas
stream is quenched prior to mixing with ozone.
20. The method as claimed in claim 13 wherein air is injected into
said process gas stream prior to entering said gas dryer.
21. The method as claimed in claim 13 wherein said absorption of
nitrous acid is in a neutral or acidic medium.
22. The method as claimed in claim 13 further comprising a
different gas-liquid contacting device with aqueous scrubbing
medium.
23. The method as claimed in claim 13 wherein said recovered
process gas stream is fed to a sulfuric acid absorption tower.
24. The method as claimed in claim 13 wherein said ozone is up to
10 weight percent ozone in oxygen.
Description
BACKGROUND OF THE INVENTION
The invention provides for processes for removing contaminants from
gas stream emissions. More particularly, the invention provides for
removing contaminants such as nitrogen oxides from gas streams in
sulfuric acid production processes.
Sulfuric acid is used in a wide spectrum of process industries.
Sulfuric acid is believed to be the world's largest chemical
produced. Over past few decades, worldwide, most of the sulfuric
acid is produced by a contact process, which involves generating a
sulfur dioxide containing gas stream from variety of sulfur
sources. Examples include burning elemental sulfur, or process of
roasting metal ore or burning H.sub.2S arising from industrial
operations such as hydrodesulphurization of petroleum products or
simply burning waste containing sulfate or sulfuric acid or
combusting spent sulfuric acid all generate SO.sub.2 in the gas
stream. If the source of sulfur is dirty, the process gas is
conditioned and oxidized to convert almost all SO.sub.2 to SO.sub.3
over a V.sub.2O.sub.5 catalyst in a multi pass converter. The
oxygen required for oxidation is either present or supplemented in
the form of additional air or oxygen. This SO.sub.3 containing
process gas stream is absorbed in sulfuric acid solution, which
results in the H.sub.2SO.sub.4 product as a >95% wt acid or
oleum of desired strength.
Since sulfuric acid is a very low cost product, and reactions are
exothermic, heavy emphasis is put on heat integration and therefore
generally most exothermic heat that is recovered is used within the
process for captive requirement of energy and any net surplus is
exported in the form of steam. Nitrogen oxides (NO.sub.X) are
generally formed during the SO.sub.2 generation step in varying
quantities based on a variety of factors. When a SO.sub.3
containing process gas stream is absorbed into a sulfuric acid
solution, some of the NO.sub.X reacts with a circulating solution
of sulfuric acid forming a complex which is referred in the
industry as niter (nitrosyl sulfuric acid) and some of its
homologs. Niter in the product is an undesirable impurity in many
applications and also imparts some color to the product.
Some of the NO.sub.X which leaves the sulfuric acid absorber passes
through much of the process equipment and is finally exhausted to
the environment. It is often noted that the plume arising from the
sulfuric acid production facility is correlated with SOx emissions,
NO.sub.X emissions, niter, types of mist eliminating devices and
various process parameters. Some of these environmental problems
are alleviated in the modern plant by a dual stage absorption
process, choosing effective mist elimination devices followed by a
caustic scrubber. Selective catalytic reduction (SCR), selective
non-catalytic reduction (SNCR) types of processes have been
suggested for NOx removal. However, the problems of NO.sub.X
emissions, acid plume, deterioration of product quality due to
niter and nitrogen containing compounds in sulfuric acid still
exists at varying levels in the industry. With increasing
environmental concern and government oversight, the present levels
of NO.sub.X controls are not adequate.
Sulfuric acid is a high production volume but a low cost and low
margin chemical. The cost of a plant producing sulfuric acid is
relatively high. The relationship of the capital cost and the plant
capacity is not linear. Therefore, plants with a larger production
capacity achieve much better scales of economy compared to plants
with smaller capacities. Sulfuric acid is a highly reactive
chemical and therefore transporting it over long distance is not
only expensive but also increasingly hazardous. For a smaller plant
operator, it makes good economic sense to boost the capacity of
sulfuric acid by employing oxygen enrichment in the SO.sub.2
generation and or oxidation stage.
Oxygen enrichment when done to the SO.sub.2 generation stage, not
only increases throughput, but also can improve thermal efficiency
thereby reducing fuel requirements, increasing SO.sub.2
concentration in the process gas stream, and exporting more steam
and reducing unit product cost. Replacing some of the
combustion/oxidation air with gaseous oxygen not only improves
capacity of the furnace but also increases SO.sub.2 content of the
process gas stream exiting the furnace. Generally downstream
equipment such as catalytic converters, waste heat recovery
equipment, fans, etc. operate more effectively at higher
concentration of SO.sub.2 and lower process gas flow rates. Typical
sulphuric acid processing equipment has adequate processing
capacity to handle 30 to 40% additional SO.sub.2 load. In the case
when SO.sub.2 is arising from a metal roasting furnace, oxygen
enrichment not only improves sulfuric acid throughput but also
enhances ore processing capacity. With all these positive aspects
of oxygen enrichment with respect to capacity and costing, there is
a major down side. Oxygen enrichment produces higher combustion
temperatures in the furnace with greater O.sub.2 concentration
resulting in higher amount of NO.sub.X formation. Without
addressing issues regarding higher environmental emissions and
increased niter content of the product, full potential or benefits
of oxygen enrichment can not be achieved. FIG. 1 depicts the
difficulty in economically justifying smaller size plants due to
longer payback period. However with O.sub.2 enrichment, this
payback period can be significantly reduced.
The sulfur dioxide containing stream when derived from sulfur acid
regeneration (SAR) plant or a metal ore processing furnace such as
a roaster furnace has high levels of particulate matter and needs
to be washed and dried prior to being oxidized to SO.sub.3.
U.S. Pat. No. 7,632,475 of common assignment herewith teaches
introducing ozone in the SO.sub.2 upstream of the particulate
scrubber to oxidize insoluble nitrogen oxides to soluble nitrogen
oxides followed by removal in a particulate scrubber addressing
both the product quality and emissions.
Although a significant portion of heat from the SO.sub.2 containing
process gas stream is recovered prior to washing in the particulate
scrubber, it is not uncommon to find the process gas exiting the
heat recovery section to be at significantly higher temperature
than that desired for nitrogen oxides oxidation with ozone upstream
of the particulate scrubber.
It is known that temperatures exceeding 100.degree. C., and in
particular 130.degree. C., performance of ozone oxidation
significantly deteriorates. Ozone molecules as well as oxides of
nitrogen higher than nitrogen dioxide decompose at elevated
temperatures. In order that ozone reacts with all nitrogen oxides
molecules, ozone must be well mixed with the bulk of the process
gas stream. Ozone must be introduced upstream far enough of the
particulate scrubber to provide the necessary mixing and reaction
time. For large volumes of the process gas, mixing time may be even
larger than the reaction time subjecting ozone and oxidized
nitrogen oxides to higher temperatures (above 130.degree. C.) for
longer periods of time. At these higher temperatures greater than
130.degree. C., ozone decomposes quickly as well as the highly
oxidized nitrogen oxides. Mixing time in industrial size equipment
is generally long and becomes a leading factor in poor performance
at higher temperatures.
Additionally, at elevated temperatures and in the presence of high
concentrations of sulfur dioxide, oxidation reactions between
sulfur dioxides and the higher oxides of nitrogen become important.
The higher oxides of nitrogen which are more soluble are wastefully
consumed in oxidizing sulfur dioxide. This is significant and this
inefficiency is enhanced with increase in the concentration of
nitrogen oxides, N.sub.2O.sub.5 and temperature as evidenced by
reactions (1) and (2) below.
SO.sub.2+NO.sub.3.fwdarw.SO.sub.3+NO.sub.2 (1)
SO.sub.2+N.sub.2O.sub.5.fwdarw.SO.sub.3+2NO.sub.2 (2)
NO.sub.2 solubility is an order of magnitude lower than
N.sub.2O.sub.5 or NO.sub.3 and limits nitrogen oxides removal in a
given particulate scrubber by wet scrubbing.
Therefore introducing ozone upstream of the particulate scrubber in
a very high concentration, hot sulfur dioxide stream as those found
in a sulfuric acid process is not a simple nor an attractive
option. Ozone requirements as described in option 1 of the '475
patent can be high. Nitrogen oxides removal may be limited and the
formation of SO.sub.3 upstream of the particulate scrubber can be a
concern.
A different approach was described in U.S. patent application Ser.
No. 12/617,356, filed Nov. 12, 2009, of common assignment herewith,
which suggested that the addition of ozone after initial
particulate scrubbing stage at a suitable location in the droplet
disengagement zone. The temperature in this zone is below
100.degree. C. and suitable for effective nitrogen oxides oxidation
with ozone.
This approach is good when nitrogen oxides concentration is low,
however oxygen enrichment in furnaces, oxy-fuel combustion and
higher furnace temperatures often result in extremely high
concentration of nitrogen oxides in sulfur dioxide containing
process gas streams. At these high concentrations of nitrogen
oxides, ozone addition will result in substantial concentrations of
N.sub.2O.sub.5 making sulfur dioxide oxidation with N.sub.2O.sub.5
even at lower temperatures relevant and undesirable.
Treating high concentrations of nitrogen oxides with ozone can also
be cost prohibitive due to the low market value of sulfuric acid.
Additionally, the large amount of nitric acid/nitrate that is
generated in the purge stream from a particulate scrubber is an
important consideration for disposal.
The invention recognizes the inefficiencies in the prior treatment
processes and provides a method for removing contaminants such as
nitrogen oxides from process gas streams resulting from sulfuric
acid production processes.
In the present invention, nitrogen oxides are only partially
oxidized by ozone. The oxidation is accomplished by adding ozone in
a sub stoichiometric quantity. Nitrogen oxides removal is
accomplished by establishing parametric conditions (Suchak et al,
1990) that enhance the formation and the absorption of nitrous acid
in the particulate scrubber or downstream equipment. However,
instead of absorbing in an alkaline medium, absorption of nitrous
acid occurs in a neutral or acidic medium. Nitrous acid in the
scrubber's aqueous phase is fed with small amounts of urea or
ammonia or other compound that can provide an ammonical radical for
the decomposition of nitrous acid. By maintaining the desired
decomposition conditions, nitrous acid decomposes in the liquid
phase to nitrogen, carbon dioxide and water. Therefore, this
invention not only requires less ozone but also minimizes nitrate
in the purge streams from the particulate scrubber or the wet
scrubber.
SUMMARY OF THE INVENTION
The invention provides for a method for removing contaminants from
a process gas stream in a sulfuric acid production process
comprising the steps: a) directing the process gas stream to a
particulate scrubber; b) contacting the process gas stream with
ozone at less than stoichiometric amounts; c) scrubbing partially
oxidized nitrogen oxides in the particulate scrubber; d)
establishing conditions under which nitrous acid is formed and
absorbed; e) feeding a compound selected from the group consisting
of urea, ammonia and a compound that can provide an ammonical
radical to the aqueous phase of the particulate scrubber; e)
directing the process gas stream to a gas dryer; and f) recovering
the process gas stream.
The invention further provides for a method for removing
contaminants such as oxides of nitrogen from a process gas stream
from a sulfuric acid production process comprising the steps: a)
feeding the process gas stream to a particulate scrubber; b)
introducing ozone into the process gas stream at less than
stoichiometric amounts with the oxides of nitrogen and scrubbing
the partially oxidized oxides of nitrogen in the scrubber, wherein
introduction of the ozone is upstream, downstream or in the
particulate washing step or the scrubber; c) establishing
conditions in the scrubber to facilitate the formation of nitrous
acid; d) feeding a compound selected from the group consisting of
urea, ammonia and a compound that can provide an ammonical radical
to the aqueous phase of the particulate scrubber; e) absorbing
nitrous acid in a neutral or acidic medium; and f) recovering the
process gas stream.
NOx removal is performed by partial oxidation followed by
absorption in a wet scrubber as nitrous acid and decomposition in
the liquid phase with urea, ammonia or a compound having or being
capable of providing an ammonical radical in the liquid phase.
Advantages of this process include cost savings due to reduced
quantity of ozone required and fewer nitrates in the scrubber purge
which has to be contended with because of environmental discharge
concerns.
In the invention, nitrogen oxides are only partially oxidized by
ozone whereas earlier processes taught almost complete oxidation of
the nitrogen oxides. The oxidation is accomplished by adding ozone
in sub-stoichiometric quantities (1/3 of what is typically needed).
If all of NO.sub.X in the process gas is in the form of NO, only
0.5 mole of ozone is required per mole of NO whereas stoichiometric
quantity to oxidize NO to N.sub.2O.sub.5 is 1.5 mole of ozone.
Nitrogen oxides removal is accomplished by establishing parametric
conditions that enhance formation and absorption of nitrous acid in
the particulate scrubber or in downstream equipment. Some of the
important parameters are humidity, temperature of the aqueous phase
or scrubbing medium and ratio of NO to NO.sub.2 (Suchak et al
1990). Instead of absorbing in the alkaline medium, the invention
claims absorption of nitrous acid in a neutral or acidic medium.
Nitrous acid in the scrubber's aqueous phase is fed with small
amounts of urea or ammonia or any other compounds that provide an
ammonical radical for decomposition of nitrous acid. The nitrous
acid decomposes in the liquid phase to nitrogen, carbon dioxide and
water by maintaining the desired decomposition conditions.
For example, conditions under which nitrous acid is formed and
absorbed are temperatures inside the particulate scrubber greater
than 40.degree. C.; a process gas stream in the scrubber that is
saturated in moisture (humidity) at 40.degree. C. or higher;
controlling the addition of the ozone to the process gas steam such
that the molar ratio of ozone to nitrogen oxides is around 0.5; and
maintaining the partial pressures of divalent nitrogen oxides and
tetravalent nitrogen oxides greater than 1 in the particulate
scrubber. These partial pressures are designated
P.sub.NO*/P.sub.NO2*>1.
The contaminants to be removed are primarily selected from the
group consisting of sulfur oxides and nitrogen oxides. The less
than stoichiometric amounts is typically one-third the amount of
ozone necessary to oxidize nitrogen oxides. Ozone is typically 10
weight percent ozone in oxygen. The process gas stream and ozone
are mixed prior to the process gas stream entering the particulate
scrubber. The process gas stream may be quenched prior to it being
mixed with the ozone.
The conditions under which nitrous acid is formed and absorbed are
in the particulate scrubber. The absorption of nitrous acid is in a
neutral or acidic medium.
The partial oxidation is performed in a process gas stream selected
from the group consisting of upstream, within and downstream of a
particulate scrubber.
A different gas-liquid contacting device with an aqueous scrubbing
medium may be used in addition to the particulate scrubber.
Air may be injected into the recovered process gas stream prior to
it entering the gas dryer.
The process gas stream from the dryer may be fed to series of heat
exchangers and SO.sub.2 to SO.sub.3 converter and then to a
sulfuric acid absorption tower.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing plant capacity for a sulfuric acid
regeneration plant versus cost recovery.
FIG. 2 is a graph showing oxygen enrichment effects on emissions
and product quality.
FIG. 3 is a schematic representation of a sulfuric acid
regeneration (SAR) process integrated with a nitrogen oxides
reduction scheme.
FIG. 4 is a schematic representation of the particulate scrubber as
used in the invention.
DETAILED DESCRIPTION OF THE INVENTION
Partial oxidation of nitrogen oxides by ozone is performed in a
process gas stream upstream, within or downstream of a particulate
washing step or aqueous scrubber.
A Sulfuric Acid Regeneration (SAR) plant and acid recovery system
on a metal ore roaster furnace is a slightly modified form of a
sulfur burning sulfuric acid plant. In all three types of plants, a
source of sulfur is converted to SO.sub.2 in the process gas. In
the first two types of plants, SO.sub.2 containing streams have
particulate matter and other acidic contaminants and need to be
washed and dried prior to oxidizing to SO.sub.3. The clean and dry
SO.sub.2 containing stream is passed through a series of heat
exchangers and beds of V.sub.2O.sub.5 catalyst to convert it to
SO.sub.3 at about 425 to 550.degree. C. Typically 3 to 4 catalyst
beds called converters are used. The heat from the process gas
stream exiting the final converter bed is used in heating the
process gas entering the converter by series of cascaded heat
exchanger. SO.sub.3 is absorbed in the sulfuric acid absorber to
form oleum or 98% sulfuric acid and some product is continuously
removed.
In the newer sulfuric acid plants, the process gas stream from the
absorber is again heated and passed through a V.sub.2O.sub.5 bed to
oxidize residual amounts of SO.sub.2 and then subjected to another
absorber to remove almost all of sulfur as SO.sub.3. The process
gas from the 2nd absorber is passed through a candle mist
eliminating device to remove H.sub.2SO.sub.4 mist and finally
scrubbed with caustic soda in an environmental scrubber before
exhausting through the stack. Environmental scrubbers are not
always employed and mostly configured in the train to meet the
local regulations governing SO.sub.2 emissions.
The main difference between a traditional sulfur burning sulfuric
acid and an acid recovery or SAR unit is how the sulfur source is
converted to SO.sub.2. A SAR unit as shown in the FIG. 3 uses a
furnace to convert spent sulfuric acid to SO.sub.2. Since
decomposition of H.sub.2SO.sub.4 is endothermic and favored by
raising the temperature, natural gas or hydrocarbon feedstock is
required to raise temperature of the furnace. Generally finely
atomized sulfuric acid is held at 650.degree. C. or higher for a
sufficient time to obtain 99.5% conversion. A supplemental feed
stream of H.sub.2S can be fed to this furnace for three primary
reasons, 1) H.sub.2S has a calorific value 2) it is a good source
of sulfur and 3) there is a monetary benefit in taking care of
H.sub.2S. The process gas stream exiting the SAR furnace, in
addition to SO.sub.2, has other contaminants, such as fly ash, etc.
After recovering the heat in the waste heat boiler, the process gas
is around 130.degree. C. to 250.degree. C. This process gas is
subjected to an aqueous wash to remove particulate matter, fly ash
and other impurities. The process gas is then dried by scrubbing
with sulfuric acid and forwarded to a series of heat exchangers and
converters.
In a conventional sulfuric acid plant, molten elemental sulfur is
burnt in a furnace to form sulfur dioxide. In contrast to SAR,
SO.sub.2 produced from elemental sulfur is relatively free from
dust, fly ash and other contaminants and does not require "washing"
or scrubbing. The SO.sub.2 containing gas stream from the furnace
can be directly led to series of waste heat boilers, converters and
heat exchangers. Therefore sulfur burning sulfuric acid plants
export as much as 1.4 tons of steam per ton of sulfuric acid
produced.
Some NO.sub.x is always produced in furnaces where SO.sub.2 is
generated. The sulfuric acid decomposition reaction in the SAR
process, in particular, is favored by higher furnace temperature
which in turn causes some of the nitrogen to convert to nitric
oxide in the furnace. Some organic nitrogen content in the spent
sulfuric acid converts to nitric oxide in the furnace. To assure
adequate destruction of organic matter in spent sulfuric acid and
for decomposition of sulfuric acid into SO.sub.2, a certain
residence time is required at furnace temperature. To increase SAR
unit throughput (up to 30%) the furnace is often supplemented with
pure oxygen stream. All these lead to formation of NOx in the
furnace.
NOx formed consists mainly of NO and a small quantity of NO.sub.2.
Both nitric oxide (NO) and nitrogen dioxide (NO.sub.2) are
sparingly soluble gases. They are not significantly removed in the
particulate scrubbers and pass along with process gas through
converters to the sulfuric acid absorber. Some of NO and NO.sub.2
reacts with the sulfuric acid and forms nitrosyl sulfuric acid
"niter" and its homologs that impart a violet coloration to the
sulfuric acid product. Industrial sulfuric acid users are sensitive
to concentrations of "nitrogen" or "niter" in the sulfuric acid.
The process gas exiting the sulfuric acid absorber still has an
equilibrium concentration of NO, some of which further condenses in
the candle mist eliminator as niter. Finally the remainder of NO
exits the sulfuric acid plant as an exhaust gas stream which is
emitted to the atmosphere via the stack.
In order to increase the production capacity in the existing SAR
furnace or metal ore kiln, the feed air can be supplemented or
partly substituted with oxygen. FIG. 2 depicts the effect of
O.sub.2 enrichment on stack emissions and product quality. As shown
in FIG. 2 with an increase in O.sub.2 concentration in the feed,
NO.sub.X content in the flue gas through stack rapidly increases
and so does the niter content of the product acid.
Therefore it is very likely that the enrichment that provides up to
30% more throughput can cause issues with the environment and
product quality. In addition, although exact reasons are not known
but higher niter content in the product acid is also associated
with visible plume at the stack.
Many geographical regions in the United States such as the
North-East, Houston-Galveston and California regions fall under
ozone non-attainment area rules and regulations. The control of
NO.sub.X emissions is a primary concern for local, state and
federal environmental protection authorities.
The Clean Air Act of 1990 and the Interstate Air Quality Rules
(IAQR) mandate the USEPA, state and local air-quality management
authorities to implement tougher standards to improve air quality.
Most existing refineries that generate spent sulfuric acid are on
the east coast, gulf coast and along the west coast of the United
States. The amount of spent sulfuric acid generated by an
individual refinery is not large enough for an economically viable
SAR unit. Therefore a separate unit that can process spent sulfuric
acid streams from more than one refinery is more preferable. Such a
unit becomes a new and independent source and therefore is outside
the bubble permit of any one refinery.
Sulfuric acid is a very low value commodity and is hazardous cargo
to haul. There is also increasing pressure on refineries to reduce
sulfur content of liquid fuels (diesel). It is therefore of
interest to set up a spent sulfuric acid plant in the vicinity of
refineries where spent and product sulfuric acid can be exchanged
via pipeline. In addition, SAR units can also advantageously
process additional amounts of H.sub.2S generated by these
refineries. However, such a location as mentioned above invites
close scrutiny in environmental permitting and mandates industry to
pursue gas pollution control devices that meet MACT standards.
Turning to the FIG. 3, a furnace A is fed through line 1 with fuel
gas. Spent acid is fed through line 2 and oxygen and hydrogen
sulfide are fed through lines 3 and 4 respectively. Process gas
from the furnace A will leave through line 13 and enter waste heat
boiler B. Steam from the waste heat boiler B will exit through line
15. The cooler process gas exits waste heat boiler B through line
14 and enters air heater C which is fed air through line 5. Hot air
from the air heater C will also be directed through line 1A into
line 1 for the fuel gas being fed to the furnace. In an alternate
configuration lines 3 and lines 4 can be also directed into line
1.
The process gas stream will leave air heater C through line 16 and
be directed into the particulate scrubber D. The first option of
the present invention begins here with the introduction of ozone
through line 12 such that the process gas stream and ozone are
mixed together prior to the process gas stream entering the
particulate scrubber D. If the process gas temperature entering the
particulate scrubber D exceeds 135.degree. C., the process gas may
be quenched prior to mixing with ozone. The scrubbed process gas
stream will exit the particulate scrubber through line 18. The
scrubbing solution 18A is pumped out of particulate scrubber D
through pump 17 and directed into the spray header assembly 17D
through line 17A. The line 17B (referring to FIG. 4) directs part
of the scrubbing solution to the nozzle or a stage that is
responsible for quenching and wetting the incoming hot process gas
stream through spray header assembly 17E.
The solution containing Urea, ammonia or compounds that can provide
ammonical nitrogen is fed in the scrubber by line 17C. The scrubber
is also fed with make up water (not shown) to maintain the liquid
level in the scrubber sump and purge line and purge to limit
concentration of dissolved and suspended solids.
The wet process gas stream in line 18 has air injected into it
through line 6 and this stream now enters the gas drying tower E.
The solution used in the gas drying tower E (generally
H.sub.2SO.sub.4) is pumped out through pump 19 and reenters the
tower via liquid distributor through line 19A. Some circulating
H.sub.2SO.sub.4 from this tower is exchanged with Sulfuric Acid
absorption tower J. This circuit is not depicted in the diagram.
Dry process gas leaves the gas drying tower E through line 20 and
this gas stream is at about 65.degree. C. This dry process gas
stream will enter a series of heat exchangers, in this example F, G
and H respectively through line 20 before entering the converter I.
Converter I has through separate converters present therein
containing catalytic materials which will convert the clean and dry
sulfur dioxide gas stream entering the converter I into sulfur
trioxide.
The sulfur trioxide generated by the catalytic conversion will exit
the converter I through line 23 and be directed to the first heat
exchanger H where it will be cooled and reenter the converter I at
a point lower than when it was removed. The same holds true with
sulfur trioxide withdrawn through line 22 where it will enter the
second heat exchanger G and reenter the converter I at a point
lower than where it was removed. Lastly the converted sulfur
trioxide is withdrawn from the bottom of the converter I through
line 21 and will pass through the third heat exchanger before it
enters the sulfuric acid absorption tower J. The heat exchange
system may also have the provision to produce steam. Oxidation of
SO.sub.2 to SO.sub.3 is highly exothermic and occurs at high
temperatures in industrial applications. Normal practice is to
carry it out in the temperature range in excess of 425.degree. C.
There are many configurations practiced in meaningful recovery and
use of heat. The present invention is applicable to all the
configurations. For the sake of brevity we have described only one
of them in this example.
Sulfuric acid is fed into the sulfuric acid absorption tower
through line 7 and the absorbing solution will exit through pump 24
through line 24A which feeds the absorbing solution into the liquid
distributor at the top of the sulfuric acid absorption tower J.
Oleum or sulfuric acid as product is withdrawn through line 8. The
gas stream which has much of its sulfuric acid content removed will
leave the absorption tower J through line 25 and enter the final
heat exchanger K before entering the final converter L. The final
converter L will contain catalytic material which will again
convert any residual sulfur dioxide into sulfur trioxide.
The process gas stream exiting the converter which now contains
little sulfur dioxide is directed through line 26 into the final
sulfuric acid absorption tower M. Sulfuric acid is circulated into
this tower. The scrubbing solution (sulfuric acid) is recovered
through pump 9A and fed back to the liquid distributor through line
9B. Some sulfuric acid (product) is also withdrawn from 9A via line
10. A sulfuric acid solution is added to absorption Tower M through
line 9. The scrubbed process gas will leave the final sulfuric acid
absorption tower through line 28 and will enter the candle mist
eliminator N. The candle mist eliminator N will contain mesh or
other gas filtering devices to separate the gas mixture entering
the eliminator which contains sulfur dioxide, some sulfuric acid
(in the form of SO.sub.3 mist), nitrogen oxides, carbon dioxide and
oxygen and nitrogen. The residual sulfuric acid which is separated
from the gas mixture will leave the candle mist eliminator through
pump 29 and be directed into feed line 10. Sometimes, the
collection from the mist eliminator is not mixed with the product
acid (in line 10) and separately processed as it may have higher
concentration of niter.
The separated gas stream which still contains nitrogen oxides will
leave the candle mist eliminator N through line 30. Ozone is
injected into this line through line 12A so that it mixes with the
process gas stream containing the nitrogen oxides before entering
the environmental scrubber O. The ozone injection spot in the line
30 is so chosen as to provide adequate residence time for ozone to
mix and oxidize NO.sub.X prior to entering the Scrubber O. Ozone is
injected via nozzle(s) or perforated tube to ensure thorough mixing
within bulk of the gas stream.
In the scrubber O the solution will scrub the nitrogen oxides and
sulfur oxides remaining in the gas mixture. Scrubber solution is
drawn from the environmental scrubber O through pump 31 and bled
from the system through scrubber bleed line 11. What solution is
not bled off is directed back into the environmental scrubber O
into its spray headers through line 31A. The process gas that is
now substantially free of nitrogen oxides and sulfur oxides will
leave the environmental scrubber O through line 32 to be directed
to the stack. The pH of the environmental scrubber is maintained by
feeding caustic soda or alkaline carbonates which is not depicted
in the FIG. 4.
When SO.sub.X, present in the line 30 is low, sulfite generated
in-situ in the environmental scrubber O may not be enough to
deplete the unreacted ozone. A small feed of sodium sulfite,
thiosulfate or reduced sulfur may also be fed to maintain sulfite
concentration in the environmental scrubber necessary to deplete
ozone.
Referring now to FIG. 4, the particulate scrubber D from FIG. 3 is
shown in larger size and greater detail.
The process gas stream will enter the particulate scrubber D
through line 16. The scrubbed process gas stream exits the
particulate scrubber through line 18. The scrubbing solution 18A is
pumped through pump 17 and line 17A to the scrubber head assembly
17D. The aqueous solution that contains urea, ammonia or compounds
that can provide ammonical nitrogen is fed into the particulate
scrubber D through line 17C.
Part of the scrubbing solution can be diverted through line 17B and
fed to spray header assembly 17E where it scan be used to quench
the incoming process gas stream through line 16 when it is
necessary to reduce the temperature of the process gas stream.
When the temperature of the process gas in line 16 is above
135.degree. C., ozone may be added after quenching using line 12B
instead of line 12.
In order to practice the process disclosed in this invention, the
quantity of ozone in the line 12 or 12B must be maintained
substoichiometric to limit the oxidation of nitrogen oxides. As
stated earlier, if all the nitrogen oxides are in the form of
nitric oxide (NO), the stoichiometric amount of ozone required to
convert NO to N.sub.2O.sub.5 is 1.5 moles of ozone per mole of
nitrogen oxides.
Nitrogen oxide oxidation to N.sub.2O.sub.5 involves several
reactions but can be represented by three reactions:
NO+O.sub.3.fwdarw.NO.sub.2+O.sub.2 (3)
NO.sub.2+O.sub.3.fwdarw.NO.sub.3+O.sub.2 (4)
NO.sub.2+NO.sub.3.fwdarw.N.sub.2O.sub.5 (5)
Reaction (3) is an order of magnitude faster compared to reactions
(4) and (5). Therefore, if the amount of ozone added is limited to
0.5 mole of ozone per mole of nitrogen oxide, the oxidation of
NO.sub.2 forming NO.sub.3 is inhibited and the resulting process
gas stream will have approximately equimolar amounts of NO and
NO.sub.2.
In the gas phase small quantities of N.sub.2O.sub.3 and
N.sub.2O.sub.4 are formed as well. NO reacts with NO.sub.2 forming
N.sub.2O.sub.3 until it reaches an equilibrium concentration.
N.sub.2O.sub.4 is also formed as a result of NO.sub.2 dimerization
reaction. NO.sub.2N.sub.2O.sub.4 (6) NO+NO.sub.2N.sub.2O.sub.3
(7)
No N.sub.2O.sub.5 formation occurs as it requires NO.sub.3
formation and since ozone is added in sub stoichiometric amounts
(Ozone/nitrogen oxides ratio around 0.5), virtually no ozone is
left in the process gas stream following the partial oxidation of
nitrogen oxide.
Both N.sub.2O.sub.4 and N.sub.2O.sub.3 possess higher solubility
compared to NO and NO.sub.2 but they are far less soluble compared
with N.sub.2O.sub.5 and removal by scrubbing in a particulate
scrubber is not an attractive option. HNO.sub.2 is far more soluble
than N.sub.2O.sub.4 and N.sub.2O.sub.3. If N.sub.2O.sub.3 (or NO
and NO.sub.2) is subjected to high water vapor in the gas phase,
appreciable amounts of nitrous acid forms. Absorption is enhanced
by maintaining a suitable NO and NO.sub.2 ratio and scrubbing at
elevated temperatures, which increases the water vapor content and
formation of nitrous acid. The sulfur dioxide process gas stream
upstream of the particulate scrubber exiting heat recovery
equipment is always substantially hot and contacting this hot
process gas in the particulate scrubber with aqueous phase raises
the temperature of the aqueous scrubbing medium in the scrubber due
to adiabatic contacting which consequently raises water vapor
content of the quenched sulfur dioxide process gas. With high
moisture content and warmer temperature in scrubbing, nitrous acid
formation is thus maximized in the gas phase.
Gas phase equilibrium is shown in reactions (8) and (9):
NO+NO.sub.2+H.sub.2O(g)2HNO.sub.2(g) (8)
N.sub.2O.sub.3+H.sub.2O(g)2HNO.sub.2(g) (9)
Due to the high solubility of HNO.sub.2, it dissolves readily in
the aqueous medium by absorption.
Absorption is presented as: HNO.sub.2(g)HNO.sub.2(I) (10)
In a gas-liquid contacting device for particulate removal, whether
it is spray, packed column or plate column, removal of nitrous acid
from the gas phase by absorption initiates formation of nitrous
acid to re-establish equilibrium in the gas phase. The formation of
nitrous acid and removal by absorption occurs simultaneously and
continuously as the process gas continues its contact with liquid
and moves along from entry to exit of the gas-liquid ontacting
device. The scrubbing medium and the process gas may contact in
co-current or counter-current fashion. (Refer to FIG. 4 for an
example)
Although a fraction of nitrogen oxides that forms nitrous acid due
to gas equilibrium reaction is small, the continuous removal makes
this approach appropriate for a particulate scrubber to
expeditiously and simultaneously remove nitrogen oxides along with
particulates.
Suchak et al. (I&EC, 1990) have shown that selectivity towards
nitrite can be greatly improved by limiting oxidation of NO.sub.X
and adjusting parametric conditions responsible for the formation
of nitrous acid. Furthermore, in the manufacture of nitrite,
absorption of NO.sub.X is enhanced by faster transport of NO and
NO.sub.2 to the gas-liquid interface in the scrubber which enhances
formation of HNO.sub.2 at the gas-liquid interface. The same
mechanism also applies for nitrous acid absorption in the acidic
medium as stated in this invention.
The aqueous scrubbing medium containing dissolved nitrous acid if
further reacted with urea, ammonia or compounds that contain
ammonia or capable of providing an ammonical radical. Urea may be
introduced either in the aqueous scrubber circulation system or
added to the purge of aqueous medium.
The reaction of nitrous acid with urea is described in equation
(11):
2HNO.sub.2(I)+CO(NH.sub.2).sub.2.fwdarw.2N.sub.2+CO.sub.2+3H.sub.2O
(11)
This reaction also needs an aqueous medium to be maintained in
acidic conditions.
In the particulate scrubber, with aqueous scrubbing, some sulfurous
and sulfuric acids are always formed in the aqueous medium due to
dissolution of sulfur dioxide and sulfur trioxide providing
suitable necessary acidic conditions for nitrous acid reaction with
urea or ammonia.
Nitrogen and carbon dioxide are released from the liquid phase and
captured nitrogen oxides are converted to nitrogen. Therefore, by
oxidation of nitrogen oxides with sub stoichiometric amounts of
ozone, converting to nitrous acid and decomposing to nitrogen, a
more advantageous nitrogen oxides treatment is realized.
Unlike nitrogen oxides oxidation with stoichiometric quantity of
ozone, the partial oxidation of nitrogen oxides does not lead to
formation of nitric acid. The partial oxidation of nitrogen oxides
has less deterioration of performance with commensurate increase in
temperature above 100.degree. C. Partial oxidation being faster,
oxidation of nitrogen oxides (nitrogen oxide to NO.sub.2) occurs
within the ozone mixing zone. Therefore, by efficiently mixing
ozone with the process gas stream, ozone can be introduced
upstream, downstream or within many of the industrially available
particulate scrubbers such as EDV scrubber offered by Belco
Technologies and Dynawave offered by MECS. When ozone is introduced
downstream of particulate scrubber, an additional gas-liquid
contacting device with aqueous scrubbing medium is required which
not only removes HNO.sub.2 formed in the gas phase but also
decomposes absorbed nitrous acid to N.sub.2.
While this invention has been described with respect to particular
embodiments thereof, it is apparent that numerous other forms and
modifications of the invention will be obvious to those skilled in
the art. The appended claims in this invention generally should be
construed to cover all such obvious forms and modifications which
are within the true spirit and scope of the invention.
* * * * *